The present invention relates to bonded abrasive products and specifically to products bonded with a vitreous bond formulation that has a surprisingly high green strength.
In producing a vitreous bonded abrasive product, such as a grinding wheel, the abrasive grits are first mixed with the bond material and the mixture is formed into the desired shape in a mold. The "green", or unfired wheel is then removed from the mold and placed in a furnace to form the bond. Because the bond material is conventionally a mixture of components that together form a glass of the desired structure when fused together, the actual firing process comprises several overlapping, sequential stages. The first stage is the melting of the components. This does not necessarily occur at the same temperature for all components. These components then must fuse together to form a homogeneous mass. This sometimes is not complete even when the bond is formed and this gives rise to uneven bond strength through the finished product. The fused bond mass must then flow till it is located essentially completely on the surfaces of the grits and in the form of bond posts joining the grits together to give the finished bonded product. The whole process typically takes several days to be completed.
Until the bond has formed there is little holding the structure together. Thus it is often difficult to remove the green wheel from the mold and to stop it from slumping during firing unless special measures are taken to prevent this. The problem is particularly acute when, as is usually the case, the green wheels are stacked in the kiln for firing. With conventional bonds one of the components is often a clay that will provide some green strength but it is usually necessary to supplement the bond with a green strength binder which is usually an organic compound such as starch or dextrin usually mixed with a glue formulation. The purpose of this is to provide temporary holding power so that the unfired product can be safely handled before it is fired. This organic binder will of course burn off as the product is fired in the kiln and this will limit the effectiveness of such additions.
The problem of lack of unfired strength is particularly acute when the vitreous bond material is a frit, that is, a pre-formed glass ground to a powder. Such bonds are described in U.S. Pat. No. 4,898,597. They have the advantage that the glass formation stage has already occurred so that the bond will have a high degree of uniformity throughout the bonded product. However any clay component of such a frit has lost its identity in the glass structure and no longer has binding power. Thus the amount of organic binder that is needed is greatly increased and at the same time, the glue component can not be used. Even with the maximum organic binder possible, the rapid loss of such material during the early stages of firing often leads to an unacceptable level of slumping in the kiln unless great care is used in the loading of the green wheels.
Thus there is a great need for a means of providing improved green strength in unfired vitreous bonded products especially where the bond is a frit bond material.